Cannot switch to mac os from windows thru bootcamp

Hi,

I set up windows 10 on my macbook pro 15 long time ago through bootcamp and I haven't used it for about one year. Recently I switched to windows 10 and found that I cannot switch back. I have tried use option key, command+r, and command+option+r+p but none of them worked. My mac starts Windows system every time no matter what combo I press while start. Any one know how to resolve this? Thank you so much!